bandejapaisa bandejapaisa - 2 months ago 14x
iOS Question

UICollectionView not removing old cells after scroll

I have a UICollectionView with a grid of images. When you tap on one, it opens up the grid and shows a subview with some details. Like this:

Supposed to look like this

I open up the grid in my UICollectionViewLayout by adjusting the UICollectionViewLayoutAttributes and setting a translation on the transform3D property for all cells below the current row of the selected item. This works really nicely, and is a much better animation and a simpler approach than my first attempt at inserting another cell into the grid which is a different size to the others.

Anyway... it works most of the time, but then after continued use I see old images on the collection view. They are like ghost cells. I can't click them, it's like they haven't been removed from the collection view properly, and they sit on top of the cells preventing taps and just being a nuisance. Like this:

Problem looks like this

Any ideas why these cells are doing this?

I'd like to add, I think it only happens when i scroll the collection view really fast. I've written my own UICollectionViewFlowLayout replacement to test if it still happens. It does.

The 3d transforms or layout have nothing to do with this. It must be a bug in UICollectionView. I can exploit by just scrolling really fast, letting come to a standstill and then querying the views that are on screen. There are often double the number of cells, but they are hidden as they are stacked on top of each other. My implementation above reveals them because of the translation i do.

This can really hurt performance too.

See my answer for a solution.


My second edit of my question details why this is happenening, and here is my workaround. It's not bullet proof, but it works in my case, and if you experience somethign similar you could tweak my solution:

- (void) removeNaughtyLingeringCells {

    // 1. Find the visible cells
    NSArray *visibleCells = self.collectionView.visibleCells;
    //NSLog(@"We have %i visible cells", visibleCells.count);

    // 2. Find the visible rect of the collection view on screen now
    CGRect visibleRect;
    visibleRect.origin = self.collectionView.contentOffset;
    visibleRect.size = self.collectionView.bounds.size;
    //NSLog(@"Rect %@", NSStringFromCGRect(visibleRect));

    // 3. Find the subviews that shouldn't be there and remove them
    //NSLog(@"We have %i subviews", self.collectionView.subviews.count);
    for (UIView *aView in [self.collectionView subviews]) {
        if ([aView isKindOfClass:UICollectionViewCell.class]) {
            CGPoint origin = aView.frame.origin;
            if(CGRectContainsPoint(visibleRect, origin)) {
                if (![visibleCells containsObject:aView]) {
                    [aView removeFromSuperview];
    //NSLog(@"%i views shouldn't be there", viewsShouldntBeThere.count);

    // 4. Refresh the collection view display
    [self.collectionView setNeedsDisplay];    


- (void) scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
    if (!decelerate) {
        [self removeNaughtyLingeringCells];

- (void) scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    [self removeNaughtyLingeringCells];